php得到一个月有多少天

function getMonthLastDay($month, $year) {
switch ($month) {
case 4 :
case 6 :
case 9 :
case 11 :
$days = 30;
break;
case 2 :
if ($year % 4 == 0) {
if ($year % 100 == 0) {
$days = $year % 400 == 0 ? 29 : 28;
} else {
$days = 29;
}
} else {
$days = 28;
}
break;
default :
$days = 31;
break;
}
return $days;
}